Astronomers using the super-sharp radio vision of the National Science
Foundation's Very Long Baseline Array (VLBA) have found the shredded
remains of a galaxy that passed through a larger galaxy, leaving only
the smaller galaxy's nearly-naked supermassive black hole to emerge and
speed away at more than 2,000 miles per second.

The galaxies are
part of a cluster of galaxies more than 2 billion light-years from
Earth. The close encounter, millions of years ago, stripped the smaller
galaxy of nearly all its stars and gas. What remains is its black hole
and a small galactic remnant only about 3,000 light-years across. For
comparison, our Milky Way Galaxy is approximately 100,000 light-years
across.

The discovery was made as part of a program to detect
supermassive black holes, millions or billions of times more massive
than the Sun, that are not at the centers of galaxies. Supermassive
black holes reside at the centers of most galaxies. Large galaxies are
thought to grow by devouring smaller companions. In such cases, the
black holes of both are expected to orbit each other, eventually
merging.

"We were looking for orbiting pairs of supermassive
black holes, with one offset from the center of a galaxy, as telltale
evidence of a previous galaxy merger," said James Condon, of the
National Radio Astronomy Observatory. "Instead, we found this black hole
fleeing from the larger galaxy and leaving a trail of debris behind
it," he added.

"We've not seen anything like this before," Condon said.

The
astronomers began their quest by using the VLBA to make very high
resolution images of more than 1,200 galaxies, previously identified by
large-scale sky surveys done with infrared and radio telescopes. Their
VLBA observations showed that the supermassive black holes of nearly all
these galaxies were at the centers of the galaxies.

However, one
object, in a cluster of galaxies called ZwCl 8193, did not fit that
pattern. Further studies showed that this object, called B3 1715+425, is
a supermassive black hole surrounded by a galaxy much smaller and
fainter than would be expected. In addition, this object is speeding
away from the core of a much larger galaxy, leaving a wake of ionized
gas behind it.

The scientists concluded that B3 1715+425 is what
has remained of a galaxy that passed through the larger galaxy and had
most of its stars and gas stripped away by the encounter -- a "nearly
naked" supermassive black hole.

The speeding remnant, the scientists said, probably will lose more mass and cease forming new stars.

"In
a billion years or so, it probably will be invisible," Condon said.
That means, he pointed out, that there could be many more such objects
left over from earlier galactic encounters that astronomers can't
detect.

The scientists will keep looking, however. They're
observing more objects, in a long-term project with the VLBA. Since
their project is not time-critical, Condon explained, they use "filler
time" when the telescope is not in use for other observations.

"The
data we get from the VLBA is very high quality. We get the positions of
the supermassive black holes to extremely good precision. Our limiting
factor is the precision of the galaxy positions seen at other
wavelengths that we use for comparison," Condon said. With new optical
telescopes that will come on line in future years, such as the Large
Synoptic Survey Telescope (LSST), he said, they will then have improved
images that can be compared with the VLBA images. They hope that this
will allow them to discover more objects like B3 1714+425.

"And also maybe some of the binary supermassive black holes we originally sought," he said.

Condon
worked with Jeremy Darling of the University of Colorado, Yuri Kovalev
of the Astro Space Center of the Lebedev Physical Institute in Moscow,
and Leonid Petrov of the Astrogeo Center in Falls Church, Virginia. The
scientists are reporting their findings in the Astrophysical Journal.

The
VLBA, dedicated in 1993, now is part of the Long Baseline Observatory.
It uses ten, 25-meter-diameter dish antennas distributed from Hawaii to
St. Croix in the Caribbean. It is operated from the NRAO's Domenici
Science Operations Center in Socorro, NM. All ten antennas work together
as a single telescope with the greatest resolving power available to
astronomy. This unique capability has produced landmark contributions to
numerous scientific fields, ranging from Earth tectonics, climate
research, and spacecraft navigation, to cosmology.

The Long
Baseline Observatory is a facility of the National Science Foundation,
operated under cooperative agreement by Associated Universities, Inc.